On the second line it is supposed to say Elokim Acherim. However the Sofer forgot to make the left leg of the Hei it now reads as a Daled and is not Kosher.
Seems like an easy fix right? Just add in the leg, a bit of ink and it will be a good Hei, it will look like nothing was ever wrong.
WRONG! One of the basic laws in writing Mezuzahs are that the letters need to be written in order, if we were to add the left leg now we would be creating the letter Hei out of order.
Why does a Sofer need to be G-d fearing? I will let you answer that question yourself, imagine you just spent nearly 4 hours writing a Mezuzah and you find this exact scenario it seems like an easy fix. Only you and G-d will ever Know if you do the right thing ( i.e dispose of it in Genizah and not make the illegal correction) as it is not discernable to the eye after the fact.
The Sofer needs to make the right decision. This is why it is important to buy only from a certified and G-d fearing Sofer / Scribe.